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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.004 Byline: Timothy D. Swindle (a), Caroline Thomas (b), Olivier Mousis (a)(b), Jonathan I. Lunine (a), Sylvain Picaud (b) Keywords: Mars; atmosphere; Mars; polar caps; Ices; Abundances; atmospheres Abstract: Calculations of the trapping of heavy noble gases within multiple guest clathrates under Mars-like conditions show that a substantial fraction of the martian Xe, perhaps even the vast majority, could be in clathrates. In addition, the Xe/Kr ratio in the clathrates would probably be much higher than in the atmosphere, so the formation or dissociation of a relatively small amount of clathrate could measurably change the atmospheric ratio. Relatively crude (factor of 2) measurements of the seasonal variability in that ratio by in situ spacecraft would be sensitive to [approximately equal to]10% of the seasonal atmospheric CO.sub.2 variability being a result of clathrates, rather than pure CO.sub.2 frost. In addition, sequestration of Xe in clathrates remains a viable mechanism for explaining the variable Xe/Kr ratios seen in different suites of martian meteorites. Author Affiliation: (a) Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA (b) Universite de Franche-Comte, Institut UTINAM, CNRS/INSU, UMR 6213, 25030 Besancon Cedex, France Article History: Received 29 January 2009; Revised 6 April 2009; Accepted 9 April 2009

Variations in the Ar/Kr/Xe ratios of martian atmosphere trapped in martian meteorites correlate with the time of ejection of these meteorites from Mars. We suggest that these variations reflect actual differences in the Ar/Kr/Xe ratios of the martian atmosphere at the time of ejection of the meteorites. We propose that the variation of the Ar/Kr/Xe ratios of the martian atmosphere is caused by time varying uptake and release of these gases by C[O.sub.2]-[H.sub.2]O clathratehydrate in the polar caps. Nakhlites, martian meteorites representing cumulate magmatic rocks, contain evidence of atmosphere from the time of their ejection from Mars. We postulate that the martian atmosphere at that time had lower Ar/Xe and Kr/Xe ratios compared with the modern atmosphere because of the release of these and other gases from polar clathrate, possibly during a period of high (~60 [degrees]) obliquity.

Abstract: Twenty-one 2–4mm rock samples from the Apollo 12 regolith were analyzed by the 40Ar/39Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800Ma, with minimum formation ages that range from 1.0 to 3.1Ga. We estimate that this degassing event occurred at 782±21Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. 40Ar/39Ar dating of two alkali anorthosite clasts yielded ages of 3.256±0.022Ga and 3.107±0.058Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing. [Copyright &y& Elsevier]

Abstract: Antarctic meteorite LAP 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments. Its petrography, oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between H and L chondrites. Petrographic indicators suggest LAP 031047 suffered a shock metamorphic overprint below ∼10GPa, which did not destroy its unusually high porosity of ∼27vol%. Metallographic textures in LAP 031047 indicate heating above ∼700°C and subsequent cooling, which caused massive transformation of taenite to kamacite. The depletion of thermally labile trace elements, the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition, and the occurrence of coarsely crystallized chondrule fragments is further evidence for post-metamorphic heating to ∼700–750°C. However, this heating event had a transient character because olivine and low-Ca pyroxene did not equilibrate. Nearly complete degassing up to very high temperatures is indicated by the thorough resetting of LAP 031047’s Ar–Ar reservoir ∼100±55Ma ago. A noble gas cosmic-ray exposure age indicates it was reduced to a meter-size fragment at <0.5Ma. In light of the fact that shock heating cannot account for the thermal history of LAP 031047 in its entirety, we test the hypothesis that this meteorite belonged to the near-surface of an Aten or Apollo asteroid that underwent heating during orbital passages close to the Sun. [Copyright &y& Elsevier]